(1) Field of the Invention
The present invention relates to a wavelength characteristic control device, a gain equalizer and a light amplifier, and more particularly, to a wavelength characteristic control device for controlling a wavelength characteristic of polarized light, a gain equalizer for actively equalizing a gain-wavelength characteristic, and to a light amplifier for amplifying signal light and actively equalizing a gain-wavelength characteristic.
(2) Description of the Related Art
The spread of optical communication networks of late years has created a demand for larger-capacity communications over a longer distance, and as a means of meeting the demand, light amplifiers and wavelength division multiplexing (WDM) are attracting attention as next-generation optical communication techniques.
FIG. 17 schematically illustrates the arrangement of a conventional light amplifier. A light amplifier 200 comprises an EDF (Erbium-Doped Fiber) 201 and a PumpLD (pumped laser diode) 202.
The EDF 201 is an optical fiber doped with Er (erbium), which is a rare-earth element, and electrons are excited to a high level by excitation light from the PumpLD 202. As signal light enters the Er atoms within the optical fiber, stimulated emission takes place and the power of the signal light is progressively amplified along the optical fiber.
The gain amplified in this case has a wavelength characteristic. Namely, the gain varies depending on the wavelength of the signal light. Thus, if light amplifiers are used directly in WDM optical transmission systems, an awkward situation arises.
For example, if light amplifiers 200 are arranged in multiple stages along a WDM transmission path, there occurs a gain difference depending on wavelength and a signal with unreceivable S/N is generated, making it impossible to perform full wavelength transmission properly.
Thus, in a conventional WDM optical transmission system having light amplifiers 200 connected thereto, a gain equalizer for flattening gain is built into each of the light amplifiers 200 or is arranged on the transmission path for every several stages of light amplifiers 200.
FIGS. 18(A), 18(B) and 18(C) illustrate gain-wavelength characteristics. FIG. 18(A) shows a cumulative gain-wavelength characteristic of a plurality of light amplifiers 200, wherein the horizontal and vertical axes indicate wavelength xcex and gain G, respectively. Let it be assumed that there is a gain difference of xcex94G between wavelengths xcex1 and xcex2, as shown in FIG. 18(A).
FIG. 18(B) shows a loss-wavelength characteristic of a gain equalizer, wherein the horizontal and vertical axes indicate wavelength xcex and lost gain G, respectively. A gain equalizer having the characteristic shown in FIG. 18(B) is inserted in the transmission path.
FIG. 18(C) shows a flattened gain-wavelength characteristic, wherein the horizontal and vertical axes indicate wavelength xcex and gain G, respectively. As shown in FIG. 18(C), the passband between the wavelengths xcex1 and xcex2 of the transmission path having the gain equalizer inserted therein shows a flattened gain-wavelength characteristic.
In this manner, when relaying signal light with the use of light amplifiers 200 which are usually arranged in multiple stages, gain equalizers having a loss-wavelength characteristic reverse to the gain-wavelength characteristic of the light amplifiers are inserted, to thereby flatten the gain-wavelength characteristic.
However, the operating point of the light amplifier 200 as described above varies with change in external factors such as environmental temperature. Also, the propagation characteristic of the transmission path varies depending on external conditions.
Consequently, the level of signal light input to the light amplifier 200 undergoes fluctuation, making it necessary to change excitation conditions so as to keep the output level constant.
Specifically, it is necessary that the PumpLD 202 emit intenser light when the input level is low and emit less intense light when the input level is high.
If, however, the power of excitation light incident on the EDF 201 varies, then the gain-wavelength characteristic of the light amplifier 200 also changes. On the other hand, the loss-wavelength characteristic of the gain equalizer is set beforehand and cannot be actively changed in response to change in the gain-wavelength characteristic.
Accordingly, if the operating point of the light amplifier 200 or the propagation characteristic of the transmission path varies, the conventional gain equalizer is unable to follow the varying gain-wavelength characteristic, thus failing to perform high-accuracy gain equalization.
As a result, the gain-wavelength characteristic cannot be flattened, giving rise to a problem that the transmission quality lowers and that only short-distance transmission is achievable.
To prevent variation of the operating point etc., the operating conditions of tie light amplifier 200 must be strictly determined taking the transmission path also into consideration, but this imposes extremely heavy restrictions on design, causing lack of flexibility.
A first object of the present invention is to provide a wavelength characteristic control device capable of variably controlling a wavelength characteristic in a satisfactory manner.
A second object of the present invention is to provide a gain equalizer which is capable of high-accuracy gain equalization and thus can improve transmission quality.
A third object of the present invention is to provide a light amplifier which performs high-accuracy gain equalization after amplification of signal light, thereby improving transmission quality.
To achieve the first object, there is provided a wavelength characteristic control device for controlling a wavelength characteristic of polarized light. The wavelength characteristic control device comprises a polarized light wavelength characteristic changing element having the wavelength characteristic such that transmittances or reflectances of P- and S-polarized rays vary differently with respect to wavelength, and polarization variable control means for subjecting a plane of polarization of the polarized light incident on the polarized light wavelength characteristic changing element to rotatory control to change a ratio of the P-polarized ray to the S-polarized ray, thereby variably controlling the wavelength characteristic.
To achieve the second object, there is provided a gain equalizer for actively equalizing a gain-wavelength characteristic. The gain equalizer comprises polarized light separating means for separating polarized signal light, polarization plane coincidence control means for making planes of polarization of a plurality of separated polarized rays coincident with each other, to thereby generate first polarized light, polarization variable control means for subjecting the plane of polarization of the first polarized light to rotatory control to change a ratio of a P-polarized ray to an S-polarized ray, a polarized light wavelength characteristic changing element for generating second polarized light having a wavelength characteristic corresponding to the changed ratio, polarization restoring means for subjecting the plane of polarization of the second polarized light to inverse rotatory control reverse to the rotatory control performed by the polarization variable control means on the plane of polarization of the first polarized light, to restore a polarized state identical with that of the first polarized light and thereby generate third polarized light, and polarized light synthesizing means for synthesizing the third polarized light.
To achieve the third object, there is provided a light amplifier for amplifying signal light and actively equalizing a gain-wavelength characteristic. The light amplifier comprises amplifying means for amplifying the signal light, polarized light separating means for separating the amplified signal light, polarization plane coincidence control means for making planes of polarization of a plurality of separated polarized rays coincident with each other, to thereby generate first polarized light, polarization variable control means for subjecting the plane of polarization of the first polarized light to rotatory control to change a ratio of a P-polarized ray to an S-polarized ray, a polarized light wavelength characteristic changing element for generating second polarized light having a wavelength characteristic corresponding to the changed ratio, polarization restoring means for subjecting the plane of polarization of the second polarized light to inverse rotatory control reverse to the rotatory control performed by the polarization variable control means on the plane of polarization of the first polarized light, to restore a polarized state identical with that of the first polarized light and thereby generate third polarized light, and polarized light synthesizing means for synthesizing the third polarized light.
The above and other objects, features and advantages of the present invention will become apparent from the following description when taken in conjunction with the accompanying drawings which illustrate preferred embodiments of the present invention by way of example.